1. Field of the Invention
The present invention relates to an apparatus and a process of coating particles, particularly particles of medicinal drugs. Particles to be coated are placed inside of a rotating drum and a coating solution or mixture is sprayed on the particles. The particles are dried and removed from the drum.
2. Description of the Prior Art
U.S. Pat. No. 3,573,966 describes a device in which tablets to be coated are introduced into a drum rotatable around a horizontal axis. The drum has a cylindrical, perforated casing or sheathing. The drum is rotated so that the tablets form a layer which is found essentially in one of the lower quadrants of the drum, and coating material is sprayed on the tablets. Subsequently, air is sucked out of the drum through the layer of tablets and through the casing by a suction socket which is adjacent to the external surface of the casing. It is not mentioned in the text of U.S. Pat. No. 3,573,966 how, or if, the air which has been sucked off is subsequently delivered to the drum. The air which has been sucked out of the drum must obviously flow afterwards above the layer of the tablets in the drum, in the course of which an air current is produced through the perforations of the casing.
The air which has been sucked out of the drum through the drum's casing strikes the tablets with a force which is added to the force of gravity and the centrifugal force. This additional force also presses the tablets against the walls of the perforated casing. Such pressure results in relatively heavy abrasion of the tablets and of the coating material applied to the tablets, which is a drawback.
When the coating material, which is to be sprayed on the tablets by a sprayer inside of the drum, has been dissolved in water or in some other solvent and is then sprayed on the tablets, the process takes place with intensive contact between the spray and the air, which is usually heated before it is delivered to the drum. As a result, a portion of the solvent is already vaporized, before the solvent and the coating material hit the tablets. Thus, a portion of the coating may be already too dry to stick firmly to the tablets by the time it reaches them. This is also detrimental.
Furthermore, it is well known in the light of U.S. Pat. No. 3,357,398, that tablets can be coated in a drum rotating around an inclined axle. The drum of this reference has not only a cylindrical portion of the wall perforated, but also conical portions of the wall on both ends of the cylindrical portion. The drum is enclosed in a housing which is surrounded by an intermediate space. This intermediate space is tightly closed and separated from the environment and is divided by packing into two uneven large parts. One of the parts, the smaller one, extends approximately through that section of the drum's circumference in which the layer of tablets is located, when the drum is in operation.
The air is brought into the drum through the perforated wall of the drum and through the layer of tablets at the smaller part of the intermediate space, and it is taken out at the other, larger section of the intermediate space, again through the wall of the drum. The disadvantage of this direction of the air stream is that light pieces, including dust formed through abrasion, are blasted upwards out of the drum. Furthermore, it is difficult to control the input of air in such a way that the supply of air has a good drying effect, without hampering the spraying of the coating material.
Another device for coating particles, particularly drugs in the form of particles, is described in International Patent Application No. PT/CH82/00071, which has been published on Nov. 25, 1982, as WO 82/03972. This application describes an apparatus similar to that provided by the present invention.